Update 2014-05-15 For examples of displaying a Web Map Service (WMS) layer in a custom projection (EPSG:27700) within Leaflet and OpenLayers 3 (ol3) see: astun-leaflet-examples and astun-ol3-examples.
Update 2012-12-18 I've update the Gist to reflect changes in v0.4 of Leaflet, in particular the fact that the scale function now belongs to the CRS object instead of the Map and the way that the TileLayer scheme is specified has changed to simply tms: true.
By default Leaflet allow you to display a map in one of the common projections used to display tiles in the Google and OSM tile schemes and the vast majority of examples you see use these. It is however possible to use another projection such as a local projection like British National Grid thanks to Proj4Leaflet which allows you to provide a Proj4 projection definition and Transformation and scale functions which map projected coordinates to grid coordinates at a given zoom.
Setting up the definition is straight forward thanks to spatialreference.org which allows you to look-up most projections and their definitions. In my case the Transformation and scale functions also turned out to be simple after a little trial and error.
I’ve been using a TMS layer provided by one of the MapProxy instances that form part of Astun Data Services, the configuration of which was as simple as providing a URL template and setting the TileLayer scheme to tms.